(1) Field of the Invention
The present invention relates to a system for dividing information that should be transferred into fixed-length cells and transferring the fixed-length cells, more specifically, to a fixed-length cell handling switching system and a method for controlling a read rate of a fixed-length cell suitable for use upon processing cells in various quality classes.
(2) Description of the Related Art
As a fixed-length cell handling switching system which handles fixed-length cells, there is an ATM switching system which switches, in particular, ATM (Asynchronous Transfer Mode) cells as fixed-length cells. ATM switching technique for applying such ATM switching system has been agreed as a next-generation switching system in ITU-T, and is being lively developed in various organizations as a technique which can realize wide-area ISDN (Integrated Services Digital Network).
The above ATM switching system uses an ATM cell which is a fixed length-cell of 53 octets as a unit for transferring information. The ATM cell has a header portion of 5 octets in which control information (destination, etc.) is stored and a data portion (payload) of 48 octets.
The ATM switching technique can transfer ATM cells asynchronously, and increase the number of ATM cells to be transferred in a given time when it is desired to transfer information at a high rate. On the other hand, when information is transferred at a low rate, the ATM switching technique decreases the number of ATM cells so as to readily regulate an information transmission rate.
By applying the above ATM switching technique, it is possible to uniformly handle information ranging from a low rate of several kbps such as voice data and the like to a high rate of several hundreds Mbps such as images and the like to transfer the information.
Since the ATM switching system handles communication services having various cell qualities such as voice, image, data, etc., a controlling system which can satisfy each quality and effectively make full use of resources of the switching system is required.
However, in a fixed-length cell handling switching system as the above ATM switching system, processes conducted in the switching system are complicated. For this, the above fixed-length cell handling switching system controls all media based on a service corresponding to data in the highest quality class among various services so that it is impossible to effectively use network resources.
In addition, since resources of the switching system are not divided in each quality class, cell discarding occurs depending on a state at that time irrespective of a grade of a quality if cell congestion occurs. Namely, congestion of cells in a low quality class may induce discarding of a cell in a high quality class. If cell congestion occurs in another communication, communication satisfying a required quality becomes impossible.